Free Radical Substitution (Cambridge International AS Chemistry)

Free Radical Substitution Mechanism

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
• Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur

Reacting an alkane with bromine

The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place

• The free-radical substitution reaction consists of three steps

Initiation step

• In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
• The covalent Cl-Cl bond is broken by energy from the UV light
• Each atom takes one electron from the covalent bond
• This produces two radicals in a homolytic fission reaction

Cl–Cl → 2Cl^•

• For more information about the initiation step, see our revision note about homolytic fission

Propagation step

• The halogen free radicals are very reactive and will attack the unreactive alkanes
• One of the methane C-H bond breaks homolytically to produce an alkyl radical

CH₄+ Cl^•→ •CH₃+ HCl

• The alkyl radical can attack another chlorine molecule to form a halogenoalkane
• This also regenerates the chlorine free radical

^•CH₃+ Cl₂→ CH₃Cl + Cl^•

• The regenerated chlorine free radical can then repeat the cycle
• For example, the chlorination of ethane is:

ethane + chlorine radical → ethyl radical + hydrogen chloride

CH₃CH₃ + Cl^•→ •CH₂CH₃ + HCl

ethyl radical + chlorine molecule → chloroethane + regenerated chlorine radical

^•CH₂CH₃ + Cl₂ → CH₃CH₂Cl + Cl^•

• This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products is formed
• If there is enough halogen present, all the hydrogens in the alkane will eventually get substituted
• For example, the chlorination of ethane could continue:

chloroethane + chlorine radical → radical + hydrogen chloride

CH₃CH₂Cl + Cl^•→ •CH₂CH₂Cl + HCl

radical + chlorine molecule → 1,2-dichloroethane + regenerated chlorine radical

^•CH₂CH₂Cl + Cl₂ → CH₂ClCH₂Cl + Cl^•

• This process can repeat until hexachloroethane, C₂Cl₆, is formed

Termination step

• The termination step is when the chain reaction terminates(stops) due to two free radicals reacting together and forming a single unreactive molecule

  • Multiple products are possible

• For example, the single substitution of ethane by chlorine can form:

ethyl radical + chlorine radical → chloroethane

^•CH₂CH₃ + Cl^•→ CH₃CH₂Cl

ethyl radical + ethyl radical → butane

^•CH₂CH₃ + •CH₂CH₃ → CH₃CH₂CH₂CH₃

chlorine radical + chlorine radical → chlorine molecule

Cl^•+ Cl^•→ Cl₂

Exam Hint If you are asked to give an equation for the termination step of a free radical reaction / mechanism, you should not give the equation reforming the original halogen as this is often marked as "ignore" on mark schemes.

Free radical substitution using bromine instead of chlorine is possible and follows similar initiation, propagation and termination steps

Further substitution

• Often, free radical reactions are not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed
• If there is enough chlorine / bromine present, all the hydrogens in the alkane will eventually get substituted
• For example, methane could be substituted to become chloromethane and then further substituted
• Single substitution:

CH₄+ Cl^•→ •CH₃+ HCl

^•CH₃+ Cl₂→ CH₃Cl + Cl^•

• Second substitution:

CH₃Cl + Cl^•→ •CH₂Cl + HCl

^•CH₂Cl + Cl₂→ CH₂Cl₂+ Cl^•

• Third substitution:

CH₂Cl₂+ Cl^•→ •CHCl₂+ HCl

^•CHCl₂+ Cl₂→ CHCl₃+ Cl^•

• Complete substitution:

CHCl₃+ Cl^•→ •CCl₃+ HCl

^•CCl₃+ Cl₂→ CCl₄+ Cl^•

Exam Hint You could be asked to draw the mechanism for initiation and termination steps for free radical substitution

This mechanism will use half-headed arrows to show the movement of one electron (double-headed arrows show the movement of a pair of electrons)

A half-headed arrow is known as a ‘fish hook’ arrow.

Initiation:

Termination:

The key is the use of the ‘fish hook’ arrow to show the homolytic fission of the bond in initiation and the formation of the bond in termination.